In recent years, small-sized devices such as mobile phones are widely used. There is a strong demand for long-lasting power with portable usage of such small-sized devices, and research is being conducted for increasing capacity of a battery used in the small-sized devices and reducing power consumption of the small-sized devices. In order to reduce the power consumption of the small-sized devices, reduction in power consumption of semiconductor devices installed in the small-sized devices is required. For example, a low-voltage power MOSFET (Metal Oxide Semiconductor Field Effect Transistor), used as a power device for the small-sized devices, has a constant internal resistance when the power is on, so that the low-voltage power MOSFET itself consumes electricity according to power current flowing therethrough. Therefore, by reducing the internal resistance in the low-voltage power MOSFET when the power is on, power consumption of the small-sized devices can be reduced. Given this, a low-resistivity N-type single crystal is strongly demanded for reducing the resistance when the low-voltage MOSFET is turned on.
A low-resistivity N-type single crystal can be obtained generally by preparing a highly-doped single crystal with an N-type dopant such as arsenic or phosphorous. However, when an ingot is pulled by the Czochralski method by doping a high concentration of an N-type dopant, the degree of solidification point depression is extremely high due to a large amount of dopant, and constitutional supercooling may be caused. When the degree of such constitutional supercooling is extreme, a growth different from that of a silicon growth face may be initiated on a crystal growth interface, which leads to an abnormal growth (cell growth). Such an abnormal growth occurring during a phase of ingot growth may inhibit single crystallization.
In view of the above, studies on such constitutional supercooling have been conducted in a field of compound semiconductor such as GaAs, InP and the like. As a result, a semiconductor in which occurrence conditions of constitutional supercooling are defined based on a relationship between a temperature gradient of the semiconductor melt and a pulling rate is known (for example, see Patent Document 1). However, since no sufficient study has been made on constitutional supercooling occurring in a silicon single crystal to which an N-type dopant is added at a high concentration, the technique cannot provide any improvement for prevention of an abnormal growth.
In addition, Patent Document 2 proposes an N-type silicon wafer of a low resistivity from 10 Ωcm to 1 mΩcm, using arsenic as a dopant. However, according to disclosures in Patent Document 2, even a silicon wafer of the lowest resistivity has a resistivity of 3 mΩcm.
Patent Document 3 discloses an N-type silicon wafer of low resistivity of no greater than 2 mΩcm, using arsenic as a dopant. Indeed, by pulling an ingot by the Czochralski method, segregation is caused, in other words the amount of dopant contained in a single crystal is low at the beginning of pulling and high at the end of the pulling. Given such a phenomenon, a low-resistivity silicon wafer can be obtained in a limited area near the bottom of an ingot; however, a low-resistivity silicon wafer thus obtained is limited in yield and production of such a low-resistivity silicon wafer is difficult from the viewpoint of cost efficiency and productivity. In addition, with regard to segregation, a highly-volatile arsenic dopant continually evaporates from a surface of a silicon melt, and dopant concentration of the ingot tends to be lower than expected due to the segregation. This also makes production of a low-resistivity wafer difficult. Actually, as disclosed in Patent Document 2, the area giving the wafer a resistivity no greater than 2 mΩcm is up to 62% of an ingot, and a top portion of the ingot does not give the wafer a resistivity no greater than 2 mΩcm. Furthermore, Patent Document 2 only discloses that the wafer with resistivity no greater than 2 mΩcm is obtained, and does not disclose specific resistivity thereof.
[Patent Document 1] Japanese Unexamined Patent Application Publication No. S61-31382
[Patent Document 2] Japanese Unexamined Patent Application Publication No. 2003-124219
[Patent Document 3] Japanese Unexamined Patent Application Publication No. 2005-314213